How to change the sorting rules in php

PHP is a widely used server-side scripting language, mainly used for web development. During development, sometimes you need to sort an array or object, and you need to know how to change the sorting rules.

PHP provides some default sorting functions, such as sort(), rsort(), asort(), arsort(), ksort(), krsort(), etc. These functions use ascending order by default and do not provide parameters to directly change the sorting rule.

If we need to change the sorting rules, we can use the usort() and uasort() functions in PHP to achieve this. The usort() function can be used to sort an array in ascending or descending order, while uasort() is used to sort an associative array according to specified rules.

Let’s take the usort() function as an example to introduce how to change the PHP sorting rules.

First, we need to define a comparison function, which is used to compare the sizes of two elements in the array, and the return value is an integer type. If the return value is less than 0, it means that the first parameter is less than the second parameter, and the first parameter is placed in front of the second parameter; if the return value is greater than 0, it means that the first parameter is greater than the second parameter, and the first parameter is placed in front of the second parameter. The first parameter is placed after the second parameter; if the return value is equal to 0, it means that the two parameters are equal and the order remains unchanged.

For example, we define the following comparison function for sorting according to string length:

function cmp($a, $b) {
    if (strlen($a) == strlen($b)) {
        return 0;
    }
    return (strlen($a) < strlen($b)) ? -1 : 1;
}

Next, we can use the usort() function to sort. The usort() function accepts two parameters, the first parameter is the array to be sorted, and the second parameter is the comparison function used for comparison. For example, we define the following array:

$arr = array("apple", "orange", "banana", "kiwi", "pear");

and then use the usort() function to sort:

usort($arr, "cmp");

Here the array is sorted according to the string length, and the final result is:

Array
(
    [0] => kiwi
    [1] => pear
    [2] => apple
    [3] => orange
    [4] => banana
)

As you can see, the array is sorted from small to large according to the string length.

It should be noted that the comparison function needs to meet certain requirements. First, it needs to accept two parameters and return an integer type value. Secondly, the comparison function must be transitive, that is, if $a is less than $b and $b is less than $c, then $a must be less than $c. Finally, comparison functions cannot change the values ​​of $a and $b.

When using the usort() function for sorting, we can also use anonymous functions to define comparison functions, which can save time and energy in defining functions. For example:

usort($arr, function($a, $b) {
    if (strlen($a) == strlen($b)) {
        return 0;
    }
    return (strlen($a) < strlen($b)) ? -1 : 1;
});

In short, changing the sorting rules in PHP is very simple, you only need to define a comparison function. Sorting can be done by using the function name as the second parameter of the usort() function. Of course, we can also use anonymous functions to simplify the code.

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